1. Field of the Invention
The present invention relates to a fluid-filled active elastic mount of active control type, which is novel in construction and which is suitably utilized as an engine mount for an automotive vehicle so as to exhibit active vibration damping effect with respect to vibration to be damped.
2. Description of the Related Art
As one type of a vibration damping device such as a vibration damping coupling (bushing) or mount, which are interposed between two members of a vibration system for flexibly connecting the two members, there is known an active elastic mount wherein a first and second support member are spaced apart from each other by a given distance and are elastically connected with each other by an elastic body which is interposed therebetween, while an oscillating force generator is disposed between the first and second support member for applying a desirably controlled oscillating force between the first and second mounting member so as to adjust vibration damping characteristics of the mount. JP-A-61-2939 discloses an example of such an active elastic mount which is operable for generating the oscillating force corresponding to the vibration to be damped, and applying the oscillating force to the member of the vibration system whose vibration to be damped, so that the vibration to be damped is actively cancelled by the applied oscillating force, and the spring characteristics of the mount is actively adjusted depending upon the vibration to be damped so as to exhibit a desirably low dynamic spring constant. Thus, the active-type vibration damping mount can enjoy an improved vibration damping capacity. The thus constructed active vibration damping device is considered to apply as an engine mount or a body mount for an automotive vehicle.
The above-described active-type vibration damping mount needs an oscillating force generator which is capable of generating an oscillating force whose frequency is suitably controllable. As disclosed in the above indicated reference, there is proposed to employ as the oscillating force generator a voice-coil type electromagnetic drive device which includes a permanent magnet having opposite magnetic pole faces which are opposite to each other and a moving coil disposed between the opposite magnetic pole faces of the permanent magnet. The moving coil is energized by a controlled electric current, whereby the coil is subjected to a Lorents force or an electromagnetic force, so that the coil is moved to provide the desirably controlled oscillating force.
However, the conventional oscillating force generator of voice-coil type is likely to generate a relatively small oscillating force. In order to generate the desired oscillating force which is large enough to assure a high damping effect, the voice-coil type oscillating force generator tends to be large sized, and the electric power consumed by the generator is inevitably increased. The voice-coil type oscillating force generator also suffers from a problem of heat generated therein. While the moving coil and the permanent magnet is displaced relative to each other in the axial direction thereof upon energizing the moving coil, the moving coil and the permanent magnet are likely to be sliding contact with each other, causing undesirable noise, loss of energy and damage to the contact parts of the moving coil and the permanent magnet.
Another type of oscillating force generator is disclosed in JP-A-10-227329 wherein an electromagnet-type oscillating force generator is employed as the oscillating force generator. Such an electromagnet-type oscillating force generator includes a yoke member made of a magnetic material and having an annular groove open in one of its axially opposite end faces, and a coil accommodated in the annular groove of the yoke member. Upon energizing the coil by application of an electric current thereto, there is generated a magnetic path or circuit around the coil, so that the inner and outer wall portions of the annular groove of the yoke member are magnetized so as to have respective opposite magnetic poles or pole faces on their open end portion. Further the electromagnet-type oscillating force generator includes an oscillating member made of a magnetic material which is opposed to the open end faces of the inner and outer wall portions of the yoke members with a given axial spacing therebetween. In this condition, the coil is energized so that the oscillating force generator generates an electromagnetic force between the oscillating member and the yoke member in the axial direction thereof. This electromagnetic force acts on the oscillating member as an axial oscillating force, causing an axially reciprocal movement of the oscillating member.
The electromagnet-type oscillating force generator permits a high-precise control of the oscillating force in terms of its frequency, phase and the like, by controlling the electric current applied to the coil. Moreover, the electromagnet-type oscillating force generator is capable of generating a sufficiently large oscillating force in comparison with the voice-coil type oscillating force generator.
In the conventional oscillating force generator of electromagnet type, however, the pole faces of the yoke member and the oscillating member are arranged to be directly opposed to each other in a direction in which these two members are displaced relative to each other, that is, in the axial direction thereof, with the predetermined axial spacing therebetween. In this arrangement, the magnitude of the oscillating force generated by the oscillating force generator is significantly influenced by an amount of spacing between the yoke member and the oscillating member. Therefore, a slight difference in an initial position of the oscillating member relative to the yoke member may cause fail in generating desired oscillating force and a resultant fail in exhibiting a sufficient vibration damping effect. Thus, the conventional oscillating force generator is incapable of exhibiting desired damping characteristics with stability.
The electromagnet-type oscillating force generator may be used in an active-type engine mount for an automotive vehicle. When the engine mount is installed on the vehicle, a load or weight of the power unit acts on the engine mount, possibly changing the axial spacing between the yoke member and the oscillating member. Therefore, it is significantly difficult to provide with high preciseness the desired spacing between the yoke member and oscillating members in the engine mount installed on the vehicle, even if the oscillating force generator is manufactured with an improved dimensional accuracy. Thus, the conventional oscillating force generator used in the engine mount suffers from further difficulty in exhibiting desired damping characteristics with stability.